Conventionally there have been proposals for laser measurement devices that use interference, on the inside of a semiconductor laser, between the laser output beam and a return beam from a measurement object (known as the “self-coupling effect”) as a way to measure a distance using interference in light from a laser. (See, for example, UEDA Tadashi, YAMADA Jun, SHITO Susumu: “Distance Measurement Using the Self-Coupling Effect of a Semiconductor Laser,” 1994 Transactions of the Institute of Electrical Engineers of Japan, Tokai Branch, 1994; YAMADA Jun, SHITO Susumu, TSUDA Norio, UEDA Tadashi: “Research Regarding Small Distance Meter Using the Self-Coupling Effect of a Semiconductor Laser,” Aichi Technical University Researcher Report, No. 31 B, Pages 35 through 42, 1996; and, Guido Giuliani, Michelle Norgia, Silvano Donati, and Thierry Bosch: “Laser Diode Self-Mixing Technique for Sensing Applications,” Journal of Optics A: Pure and Applied Optics, Pages 283 to 294, 2002.) A compound resonator model for an FP-type (Fabry-Perot-type) semiconductor laser is illustrated in FIG. 10. In FIG. 10, 101 is a semiconductor laser, 102 is a said opening surface of a semiconductor crystal, 103 is a photodiode, and 104 is a measurement object.
When the emission wavelength of the laser is λ, and the distance from the side opening surface 102 nearest to the measurement object 104 to the measurement object 104 is L, then when the resonance conditions set forth below are fulfilled, the return beam from the measurement object 104 and the laser beam within a resonator 101 reinforce each other increasing slightly the laser power:L=qλ/2  1In Equation (1), q is an integer. This phenomenon produces an amplifying effect, which is fully observable, through increasing the apparent reflectance rate within the resonator 101 of the semiconductor laser, even if the scattered light from the measurement object 104 is extremely weak.
Because a semiconductor laser emits a laser beam of a frequency that varies according to the magnitude of the injected electric current, there is no need for an external modulator when modulating the emission frequency, enabling direct modulation through the injected electric current. FIG. 11 is a diagram illustrating the relationship between the emitted wavelength and the output waveform of a photodiode 103 when varying the emission wavelength of the semiconductor laser with a constant ratio. When the L=qλ/2 given in Equation (1) is fulfilled, then the phase difference between the return beam and the laser beam within the resonator 101 goes to 0° (that is, the phase is the same), causing maximum mutual reinforcement between the return beam and the laser beam within the resonator 101, and when L=qλ/2+λ/4, the phase difference will be 180° (the phase will be inverted), so the return beam and the laser beam within the resonator 101 weaken each other. Because of this, when the emission wavelength of the semiconductor laser is changed, the laser power is seen to repetitively alternate between places wherein it becomes stronger and places wherein it becomes weaker, and if the laser power at this time is detected by a photodiode 103 provided in the resonator 101, a waveform is obtained that has a stair-step shape at regular intervals, as illustrated in FIG. 11. This type of waveform is typically called interference fringes.
In this stair-step waveform, each of the individual interference fringes is known as a “mode hop pulse” (hereinafter termed an “MHP”). An MHP is a different phenomenon from the mode hopping phenomenon. If, for example, the number of MHPs is 10 when the distance to the measurement object 104 is L1, then the number of MHPs would be 5 at half the distance L2. That is, if the emission wavelength of the semiconductor laser were varied over a given time period, the number of MHPs would vary proportionately with the measurement distance. Consequently, measuring the MHP frequency by detecting MHPs using the photodiode 103 makes it easy to measure the distance.
However, in a conventional interferometric measuring device that includes self-coupling, there is a problem in that it is not possible to measure the distance of a measurement object that has a velocity, despite being able to measure the distance to a stationary measurement object.
Given this, the present inventors have proposed a distance/velocity meter capable of not only measuring the distance to a stationary measurement object, but also measuring the velocity of the measurement object. (See Japanese Unexamined Patent Application Publication No. 2006-313080, hereinafter “JP '080”.) FIG. 12 illustrates the structure of this distance/velocity meter. The distance/velocity meter of FIG. 12 includes an a semiconductor laser 201 for emitting a laser beam towards the measurement object; a photodiode 202 for converting the optical power of the semiconductor laser 201 into an electric signal; a lens 203 for not only focusing the beam from the semiconductor laser 201 and directing the beam towards the measurement object 210, but also for focusing the return beam from the measurement object 210 and injecting it into the semiconductor laser 201; a laser driver 204 for repetitively alternating between a first emitting period wherein the emission wavelength of the semiconductor laser 201 is continuously increased, and a second emitting period wherein the emission wavelength is continuously decreased; a current-voltage converting amplifier 205 for converting the output current of the photodiode 202 into a voltage and then amplifying; a signal extracting circuit 206 for performing double differentiation of the output voltage of the current-voltage converting amplifier 205; a counting circuit 207 for counting the number of MHPs included in the output voltage of the signal extracting circuit 206; a calculating device 208 for calculating the distance to the measurement object 210 and the velocity of the measurement object 210; and a displaying device 209 for displaying the calculation results of the calculating device 208.
The laser driver 204 provides, to the semiconductor laser 201, a triangle wave driving electric current, as an injection current, that is repetitively increased and decreased at a constant rate of change in respect to time. As a result, the semiconductor laser 201 is driven so as to repeatedly alternate between a first emission period wherein the emission wavelength increases continuously at a constant rate of change, and a second emission period wherein the emission wavelength decreases continuously at a constant rate of change. FIG. 13 is a diagram illustrating the change in the emission wavelength of the semiconductor laser 201 over time. In FIG. 13, P1 is the first emission period, P2 is the second emitting period, λa is the minimum value for the emission wavelength in each of the periods, λb is the maximum value for the emission wavelength in each of the periods, and P0 is the period of the triangle wave.
The laser beam that is emitted from the semiconductor laser 201 is focused by the lens 203 and is incident on the measurement object 210. The beam that is reflected from the measurement object 210 is focused by the lens 203 and is injected into the semiconductor laser 201. The photodiode 202 converts the optical power of the semiconductor laser 201 into an electric current. The current-voltage converting amplifier 205 converts the output current of the photodiode 202 into a voltage and amplifies the results. The signal extracting circuit 206 performs double differentiation on the output voltage of the current-voltage converting amplifier 205. The counting circuit 207 counts the number of MHPs included in the output voltage of the signal extracting circuit 206 over the first emission period P1 and the second emission period P2. The calculating device 208 calculates the distance to the measurement object 210 and the velocity of the measurement object 210 based on the minimum emission wavelength λa and the maximum emission wavelength λb of the semiconductor laser 1, the number of MHPs in the first emission period P1, and the number of MHPs in the second emission period P2.
The self coupling-type distance meter illustrated in FIG. 10 is capable of measuring the distance to a measurement object, and the distance/velocity meter illustrated in FIG. 12 is capable of simultaneously measuring the distance to a measurement object and the velocity of the measurement object.
However, in a conventional self-coupling-type laser measurement instrument illustrated in FIG. 10 and FIG. 12, the emission wavelength of the semiconductor laser is varied in the form of a triangle wave, so they have the problem of not being able to completely exclude the effect of transient responses at the apexes of the triangle wave. FIG. 14(A) and FIG. 14(B) are diagrams for explaining the problem area with the conventional self-coupled-type laser measurement device, where FIG. 14(A) is a diagram illustrating schematically the output voltage waveform of the current-voltage converting amplifier 205, and FIG. 14(B) is a diagram illustrating schematically the output voltage waveform of the signal extracting circuit 206.
The signal extracting circuit 206, which is structured from a differentiating circuit or a high-pass filter, excludes the emission waveform (carrier wave) of the semiconductor laser 1 of FIG. 13 from the waveform (modulated wave) of FIG. 14(A) that corresponds to the output of the photodiode 202, to extract the MHP waveform of FIG. 14(B). At this time, a spike-shaped transient waveform, such as in FIG. 14(B), appears with the timing of the apexes of the triangle wave, in the output of the signal extracting circuit 206. Because the counting circuit 207 cannot count the MHPs in the transient waveform portion, error is produced in the count. The result is that there will be error in the distance and the velocity calculated by the calculating device 208.
The present invention was created in order to solve the problem area set forth above, and the object thereof is to provide a physical quantity sensor and physical quantity measuring method capable of reducing the effect of the transience at the triangle wave apex points that is included in the output signal of the photoreceiver device.
Summary of the Invention The physical quantity sensor as set forth in the present invention has a semiconductor laser for emitting a laser beam at a measurement object; a laser driver for supplying to the semiconductor laser a driving electric current with a waveform wherein maximum portions and minimum portions of a triangle wave are rounded; detecting means for detecting an electric signal that includes an interference waveform that is produced by the self-coupling effect of the laser beam that is emitted from the semiconductor laser and the return beam from the measurement object; and measuring means for measuring a physical quantity of the measurement object from interference waveform information that is included in an output signal from the detecting means.
Additionally, in an example of the physical quantity sensor in the present invention, the waveform of the driving electric current is a waveform wherein maximum portions and minimum portions of the triangle wave are replaced by maximum portions and minimum portions of a sine wave.
Additionally, in an example of the physical quantity sensor in the present invention, the waveform of the driving electric current is a waveform that combines a sine wave of a fundamental frequency and a sine wave of a higher-order frequency.
Additionally, in an example of the physical quantity sensor in the present invention, the physical quantity of the measurement object is a distance from the measurement object and/or a velocity of the measurement object.
Furthermore, in an example of the physical quantity sensor according to the present invention, the measuring means includes counting means for counting the number of interference waveforms included in an output signal of the detecting means over a counting period on the rising side of the semiconductor laser emission wavelength and over a counting period on the falling side of the emission wavelength; and calculating means for calculating a distance from the measurement object and/or a velocity of the measurement object from the counting results of the counting means and the minimum emission wavelength and the maximum emission wavelength of the semiconductor laser.
Furthermore, the physical quantity measuring method according to the present invention includes an emitting process wherein a driving electric current that has a waveform wherein maximum portions and minimum portions of a triangle wave are rounded is supplied to a semiconductor laser to cause the semiconductor laser to operate; a detecting process for detecting an electric signal that includes an interference waveform that is produced by the self-coupling effect between a laser beam that is emitted from the semiconductor laser and a return beam from the measurement object; and a measuring process for measuring the physical quantity of the measurement object from interference waveform information that is included in the output signal obtained in the detecting process.
Given the present invention, a driving electric current of a waveform wherein maximum portions and minimum portions of a triangle wave are rounded is supplied to a semiconductor laser, so that when an information for the interference waveform that is included in the output signal of the detecting means is obtained, the transient waveform that is produced with the timing with which the rising portion and the falling portion of the output signal of the detecting means switches can be reduced relative to the conventional technology, making it possible to reduce the interference waveform detection error. The result is that, in the present invention, it is possible to improve the measurement accuracy of the physical quantity.